1. Field of the Invention
The present invention relates to an apparatus and method for performing thin film layer thickness metrology and, more particularly, to an apparatus and method for performing thin film layer thickness metrology on a silicon/silicon dioxide/silicon (Si/SiO.sub.2 /Si) structured semiconductor wafer by illuminating the wafer with a diffuse light source.
2. Description of the Prior Art
In one particular application wherein the present invention is especially practical, a silicon-on-insulator (SOI) semiconductor wafer, typically consisting of an Si/SiO.sub.2 /Si sandwich structure, is fabricated by growing a silicon dioxide (SiO.sub.2) film on one surface of each of two silicon (Si) wafers and bonding the two silicon dioxide film surfaces together at high temperature. This fabrication process usually results in deformations in the shape of the SOI wafer that typically range from 50 to 100 microns. The SOI wafer then undergoes further fabrication processing, wherein an outer surface of one of the two silicon wafers in the sandwich structure is mechanically ground and polished to an average thickness of several microns. This mechanical grinding and polishing unfortunately results in large spatial variations in the thickness of this one silicon wafer, or this one outer silicon layer. These spatial variations can result in local slope variations of up to 1/4 of a degree along the surface of this outer silicon layer. To reduce these spatial variations, a thickness error map that indicates thickness non-uniformities in this outer silicon layer over the entire wafer surface is required, for example, to initialize a subsequent micropolishing process.
A sequence of measuring the spatial variations in the thickness of the outer silicon layer followed by thinning and smoothing the surface of this layer by micropolishing may need to be performed several times before the entire outer silicon layer achieves a desired thickness. In order to reduce costs and increase production, a measurement of at least 400 points on a wafer surface in 60 seconds is desirable.
Current commercial instruments, however, can typically provide a thickness measurement of a thin film layer at only a single point thereon. These instruments use a focused lens or a fiber bundle to locally illuminate a surface of the thin film layer with a beam of monochromatic light, and a grating or prism spectrograph to measure the surface spectral reflectance at each point. In all cases, this surface spectral reflectance data must be numerically corrected due to variations in the angle of incidence caused by the f-,, number of the illuminating beam.
These current commercial instruments may be extended to cover an entire thin film layer, such as an outer silicon layer of an SOI semiconductor wafer, by moving either the measuring instrument or the wafer in a controlled manner. However, the time required for these instruments to determine the thickness of an outer silicon layer of an SOI semiconductor wafer at a single point thereon is on the order of several minutes, and characterizing the entire outer silicon layer with at least 400 measurement points far exceeds the time desired for efficient wafer production. It is therefore desirable to perform thin film layer thickness metrology over an entire thin film layer, such as an entire outer silicon layer of an SOI semiconductor wafer, in an efficient, albeit an economical and highly accurate manner.